A belt type continuously variable transmission in which a metal belt is wound around a drive pulley provided on an input shaft and a driven pulley provided on an output shaft, and transmission is carried out by changing the groove width of the two pulleys using transmission oil pressure, wherein a μ gradient, which is the rate of change of the coefficient of friction relative to the slip velocity of the metal belt, is calculated, and when the μ gradient attains a predetermined threshold value or below it is determined that a large amount of slippage has occurred in the metal belt, is known from Patent Document 1 below.
Furthermore, an arrangement is known from Patent Document 2 below in which the power transmission efficiency of a belt type continuously variable transmission is enhanced by taking into consideration that the power transmission efficiency is closely related to a parameter called the torque ratio r, which is obtained by dividing the torque actually transmitted between the input shaft and the output shaft by the maximum torque that can be transmitted without causing slippage in the metal belt, and since the maximum power transmission efficiency is obtained in a state in which the torque ratio r=1, focusing attention on a difference in the amplitude or a difference in the phase of a variable component (variation in rotational speed or variation in torque) of the rotational state between the input shaft and the output shaft due to slippage of the metal belt, parameters such as a slip identifier IDslip and a phase lag Δϕ, which are indicators for the torque ratio r, are introduced, and the pulley thrust is controlled so that the slip identifier IDslip and the phase lag Δϕ coincide with a slip identifier reference value IDslip′ and a phase lag reference value Δϕ′ corresponding to the torque ratio r=1.